This invention relates to flat products such as strip or sheet (hereinafter referred to simply as "strip") comprising at least two bonded layers of metallic or non-metallic material of different chemical composition.
The invention has application to bi-metallic or tri-metallic strip intended for use as temperature sensitive elements in thermostats, circuit breakers and the like devices which produce mechanical movement or develop a force in response to temperature change. The invention is, however, not limited to such application and is applicable to the production of a wide range of multi-layered strips where the properties of one surface or layer are required to be different from those of other surface(s) and/or layer(s). Thus, one surface or layer may act as a catalyst or react chemically with another surface or layer when the strip is subjected to specified conditions.
It is known to produce multiple layer strip by welding or otherwise bonding together two or more relatively thick metal slabs and progressively reducing the combined thickness of the contiguous bonded slabs by hot or cold rolling, a heat treatment where required being interposed between working operations. This method for producing strip in addition to being costly requires a high level of care and displays a relatively high reject rate which adds further to the cost of strip having the required characteristics. In particular, it is well known that a high standard of surface cleanliness is required and that, consequently, the available time between cleaning and bonding is very limited, in some cases a matter of seconds. In addition, extremely high rolling pressures are required to achieve the 50% or more reduction in thickness needed in one roll pass to promote roll bonding between the layers, each of which is 100% dense.
It is also known to produce strip from metal powder in which a coating of a slurry comprising a suspension of metal powder in a binder composition is deposited onto a support surface, the slurry being dried, removed from the support surface and rolled to form a green strip. Hitherto, however, it has proved to be impracticable to produce a multiple layer strip from such strips due to difficulties in achieving a consistent and satisfactory interfacial contact between superimposed strips and an inability to produce at reasonable cost a satisfactory mechanical bond between the strips. The former difficulty arises because of a tendency for one or each strip to move in a direction parallel to the axes of rotation of the compaction rolls.